Fish is a sign of the first Christians. What is its name in the Old Greek language?

The ichthys, from the Greek ikhthýs is a symbol consisting of two intersecting arcs, the ends of the right side extending beyond the meeting point so as to resemble the profile of a fish. The symbol was adopted by early Christians as a secret symbol. It is now known colloquially as the "sign of the fish" or the "Jesus fish".